Jl. Swenson et Sl. Beck, HISTORICAL 1942 ECUADOR AND 1942 PERU SUBDUCTION EARTHQUAKES, AND EARTHQUAKE CYCLES ALONG COLOMBIA ECUADOR AND PERU SUBDUCTION SEGMENTS, Pure and Applied Geophysics, 146(1), 1996, pp. 67-101
Two large shallow earthquakes occurred in 1942 along the South America
n subduction zone in close proximity to subducting oceanic ridges: The
14 May event occurred near the subducting Carnegie ridge off the coas
t of Ecuador, and the 24 August event occurred off the coast of southw
estern Peru near the southern flank of the subducting Nazca ridge. Sou
rce parameters for these two historic events have been determined usin
g long-period P waveforms, P-wave first motions, intensities and local
tsunami data. We have analyzed the P waves for these two earthquakes
to constrain the focal mechanism, depth, source complexity and seismic
moment. Modeling of the P waveform for both events yields a range of
acceptable focal mechanisms and depths, all of which are consistent wi
th underthrusting of the Nazca plate beneath the South American plate.
The source time function for the 1942 Ecuador event has one simple pu
lse of moment release with a duration of 22 seconds, suggesting that m
ost of the moment release occurred near the epicenter. The seismic mom
ent determined from the P waves is 6-8 x 10(20) N . m, corresponding t
o a moment magnitude of 7.8-7.9. The reported location of the maximum
intensities (TX) for this event is south of the main shock epicenter.
The relocated aftershocks are in an area that is approximately 200 km
by 90 km (elongated parallel to the trench) with the majority of after
shocks north of the epicenter. In contrast, the 1942 Peru event has a
much longer duration and higher degree of complexity than the Ecuador
earthquake, suggesting a heterogeneous rupture. Seismic moment is rele
ased in three distinct pulses over approximately 74 seconds; the large
st moment release occurs 32 seconds after rupture initiation. The seis
mic moment as determined from the P waves for the 1942 Peru event is 1
0-25 x 10(20) N . m, corresponding to a moment magnitude of 7.9-8.2. A
ftershock locations reported by the ISS occur over a broad area surrou
nding;he main shock. The reported locations of the maximum intensities
(TX) are concentrated south of the epicenter, suggesting that at leas
t part of the rupture was to the south. We have also examined great hi
storic earthquakes along the Colombia-Ecuador and Peru segments of the
South American subduction zone. We find that the size and rupture len
gth of the underthrusting earthquakes vary between successive earthqua
ke cycles. This suggests that the segmentation of the plate boundary a
s defined by earthquakes this century is not constant.